Recent advances in silicon chip technology have increased the need for multilayer interconnect systems to provide higher density circuits, faster signal propagation and to allow direct silicon die attachment. To meet these new requirements, the dielectric material must have a low dielectric constant (preferably less than 5) to reduce signal propagation delay, and it must have a thermal expansion coefficient close to the value for silicon (3.5 ppm/.degree.C.) to allow direct die attachment to the substrate.
Heretofore, most of the dielectric materials used in multilayer circuits have been conventional thick film compositions. A typical circuit is constructed by sequentially printing, drying and firing functional thick film layers atop a ceramic substrate which is usually 92-96% wt. Al.sub.2 O.sub.3). The multiple steps required make this technology process intensive with the large number of process steps and yield losses attributing to high costs. Thick film technology nevertheless fills an important need in microelectronics and will continue to do so in the foreseeable future. Recently, dielectric thick film compositions with low dielectric constant have been introduced. However, ceramic substrates with thermal expansion coefficients equal to that of silicon (3.5 ppm/.degree.C.) are not readily available.
High temperature co-fired (HTCF) technology has been used to fabricate multilayer circuits and offers processing advantages over thick film. This tape process involves lamination of a number of thin sheets of ceramic dielectric material (usually Al.sub.2 O.sub.3) interspersed with alternating printed layers of conductive material. However, co-firing alumina with metallization requires a 1600.degree. C. reducing environoments with refractory metals such as molybdenum or tungsten, both of which have poor conductivity relative to metals available for use below 1000.degree. C.
Low temperature co-fired (LTCF) technology has been recently introduced as a method for fabricating multilayer circuits. This technology offers the combination of the processing advantages of HTCF technology and the materials advantages of thick film technology. These LTCF tape systems have firing temperatures below 1000.degree. C. and allow the use of high conductivity metals such as silver, gold, platinum and copper. (Copper, however, requires reducing atmospheres.) Most of these tape systems have dielectric constants between 6 and 8 and encompass a range of thermal coefficient of expansion (TCE). Currently, there is no readily available low temperature co-fired dielectric tape system that offers both low dielectric constant (less than 5) and a TCE matched to silicon (3.5 ppm/.degree.C.).
From the foregoing, it can be seen that there is a substantial need for a low temperature co-fireable tape dielectric which (1) has a low dielectric constant (less than 5), (2) has a thermal expansion coefficient very close to the value for silicon (3.5 ppm/.degree.C.), and (3) can be fired in air at a low temperature (less than 1000.degree. C.), thus permitting the use of high conductivity metallurgies such as gold, silver and silver/palladium.